The invention relates to a method for producing surface acoustic wave sensors on the basis of surface acoustic wave components and to a surface acoustic wave sensor.
The use of surface acoustic wave components as gas sensors was originally proposed by Wohltjen who examined this measuring technique(Wohltjen, H., Dessey, R.: Surface acoustic wave probe for chemical analysisxe2x80x9d; Anal. Chem. 51 (1979) 1458-1464). These Surface Wave (OFW) building elements which were actually developed as miniaturized high frequency filters find increasing interest for use as chemical and biochemical sensors (Rapp, N.; Barie, N.; Stier, S.; Ache, H.J.; xe2x80x9cOptimization of an analytical SAW microsystem for organic gas detectionxe2x80x9d; Proc. IEEE Ultrasonic. Symp. (1995) 477-490). As sensor property, the mass-sensitive behavior of such components is utilized by applying a coating which is selective for the material to be sensed and the component is the frequency determining element of an oscillator resonant circuit.
In a surface wave component, interdigital transducers are disposed on the piezo-electric substrate of the building component. By the application of a high frequency AC voltage, the transmitter transducer is caused to vibrate. The surface wave generated thereby moves over the substrate and is converted, by the piezo effect, in the receiver transducer into an electrical alternating field. By way of an amplifier, which compensates for the losses resulting from the attenuation of the acoustic wave, the electrical signal is again fed into the transmitter transducers. In this way, an oscillation with a specific resonance frequency, which is based on the travel time of the acoustic and electric wave, occurs in the oscillation circuit. This frequency is uncoupled from the circuit as a measuring signal.
The propagation velocity of the surface wave depends on the character of the surface. If the piezo electric substrate is provided with a thin selective coating the acoustic velocity is changed and, as a result, the resonance frequency of the oscillator circuit is changed.
If an analyte sample is applied to the coated component, a sorption of the analyte occurs in the layer whereby the mass of the building component is increased. As a result, the acoustic velocity is again changed which results in a measurable change of the oscillation frequency. Since frequencies can be measured very accurately, already very small changes of the mass charge of the surface wave building component can be detected.
During the coating of the surface wave sensor with a viscous sorption polymer, the cross-linkages of the sensor surface are often destroyed by the sorption polymer. This results in a non-uniform sorption polymer layer on the substrate and, because of an excessive attenuation caused thereby, in a drastic deterioration of the sensor sensitivity.
It is the object of the present invention to provide a method, which permits the manufacture of a homogeneous sorption polymer layer and a sensor with a homogeneous sorption polymer layer.
In a method for producing a surface wave sensor on the basis of a surface wave building component a polymer parylene film with a thickness of 20 to 200 nm is applied to a hydrophilic sensor surface of the surface wave building component by deposition from the gas phase, whereby the hydrophilic sensor surface becomes hydrophobic, the surface is then subjected to plasma activation to render it hydrophilic and a hydrophilic sorption polymer layer is then applied to the parylene film by spray coating or drip coating.
The invention has the following advantages:
Because of the better wetting of the sensor surface by the sorption polymers, less complicated coating techniques such as drip coating can be utilized. The improved wetting results in a more uniform deposition of the sorption polymer and, consequently, in a lower attenuation than can be obtained with a non-uniform layer of the same amount of the same polymer. The sensitivity of the sensor can therefore be increased by increasing the layer thickness of the sorption polymer up to a critical attenuation. With the improved wetting also the aging behavior of the sensor is positively affected.
Below, the invention will be described in greater detail on the basis of examples with reference to the drawings.